Colonial Microcystis’ biomass affects its shift to diatom aggregates under aeration mixing

The effect of hydrodynamic mixing on controlling Microcystis blooms or changing the algal community to diatom dominance has been widely studied; however, the effects of colonial Microcystis biomass on the development of the algal community are poorly known. Here, in order to study the changes in Microcystis blooms under continuous aeration mixing, an experiment was carried out in a greenhouse with factors of varying biomass of Microcystis and inorganic nitrogen and phosphorus enrichment in summer. There were three chlorophyll a (Chl-a) levels in six treatments: low Chl-a level of 68.4 μg L-1 (treatments L, L-E), medium Chl-a level of 468.7 μg L-1 (treatments M, M-E), and high Chl-a level of 924.1 μg L-1 (treatments H, H-E). Treatments L-E, M-E and H-E were enriched with the same inorganic nitrogen and phosphorus nutrients. During the experiment of 30 days, the concentration of Microcystis and Chl-a decreased, and diatom Nitzschia palea cells appeared in all the treatments, which became dominant in treatments M, M-E, H and H-E, with the highest biomass of 9.41 ± 1.96 mg L-1 Nitzschia in treatment H-E on day 30. The rank order of the biomass of Nitzschia from low to high was (L = L-E) < (M = M-E) < H < H-E (P < 0.05). In addition, Nitzschia cells were aggregates attached to Microcystis colonies in all the treatments. The results showed that the initial biomass of colonial Microcystis affected the algal shift from Microcystis dominance to Nitzschia dominance. However, the enriched inorganic nitrogen and phosphorus was beneficial for the Nitzschia increase in the high biomass treatment alone. The shift from Microcystis dominance to diatom dominance under continuous aeration mixing may be caused by low light conditions as well as the nutrients released from Microcystis decay. Moreover, the aerobic condition caused by aeration mixing maintained the colonial mucilaginous sheath to support the growth of Nitzschia cells in aggregation. This study found for the first time that Microcystis blooms could shift to diatom Nitzschia dominance in aggregates. It provided a method to control and manipulate Microcystis blooms to diatom dominance through continuous aeration mixing to proper biomass of Microcystis colonies. The shift to diatoms dominance would provide more high quality food organisms for aquaculture and be beneficial to the material cycling and energy flowing in food web dynamics.

The obvious feature of Microcystis bloom is that it consists of fine particles of Microcystis colonies, which can be up to several millimeters and float on the water instead of being suspended in the water column if the hydrodynamic force is not strong enough.And the algal shift from Microcystis dominance to diatoms or diatoms and green algae dominance was related to such reasons: (a) Microcystis colonies lose their advantage of buoyancy of floating on the water under artificial turbulence 21 ; (b) The negatively buoyant algae, such as green algae and diatoms, profited from the mixed conditions with fluctuating irradiance 18,21 .Visser et al. 22 reviewed the control of cyanobacteria blooms by artificial mixing, including the algal shift mechanisms.
However, the opinion that artificial mixing was an effective solution to control harmful Microcystis blooms was not unanimous, and that the hydrodynamic disturbance influencing blooms has not been fully understood 23 .Hydrodynamic disturbance may have a completely different impact on the Microcystis dominance, which may improve Microcystis dominance formation.For example, the tropical cyclones stimulated Microcystis blooms in hypertrophic Lake Taihu, China 24 .Moreover, the disturbance frequency and intensity on the algal shift process may have varying effects 25,26 , in which continuous hydrodynamic mixing weakened the dominance of Microcystis 26 and intermittent disturbance benefited colony size, biomass, and dominance of Microcystis in Lake Taihu under simulated field conditions 25 .
Meanwhile, the abundance of Microcystis can be an important factor affecting the algal shift under hydrodynamic disturbance 21 .Our initial study showed that the biomass of bloom-forming colonial Microcystis affected its response to aeration disturbance, in which the biomass of diatom Nitzschia was the highest when the initial chlorophyll a (Chl-a) of the bloomed Microcystis was 346.8 μg L -1 , in comparison with those of 32.5, 1413.7, and 14,250.0μg L -1 , respectively 27 .Then the interval between 346.8 and 1413.7 μg L -1 was so large, what would happen to the Microcystis colonies if the biomass level was between 346.8 and 1413.7 μg L -1 ?There may be more diatom Nitzschia shift from the Microcystis colonies if the Microcystis colonies' biomass level was between 346.8 and 1413.7 μg L -1 in Chl-a.Then we hypothesized that the initial Microcystis biomass can affect the algal shift under aeration mixing, and there was a more suitable Microcystis biomass level to achieve the shift from Microcystis dominance to diatom dominance.
In the review of Visser et al. 22 , the effect of the biomass of colonial Microcystis on the algal shift was not included, although the effects of artificial mixing on algal biomass changes were reviewed.The algal succession in Microcystis blooms or dominance of different biomass under hydrodynamic disturbance was not fully understood, as many factors affect the shift process, such as nutrients, buoyancy regulation, temperature, oxygen, light, and so on 22 .Moreover, biomass can change the nutrients level, which can affect the algal community composition [28][29][30] .
In order to obtain a more suitable range of Microcystis biomass that can promote more diatom succession and clarify if inorganic nitrogen and phosphorus nutrients enrichment can improve the algal shift, an experiment with varying biomass of colonial Microcystis, particularly the Chl-a level between 346.8 and 1413.7 μg L -1 , coupling inorganic nitrogen and phosphorus enrichment, was carried out in a greenhouse, which can provide high temperature to improve the algal shift process.Then the mechanism of the algal shift from Microcystis dominance to other algae dominance would be clearer.
Moreover, diatoms, different from Microcystis containing microcystins, can produce high-value compounds, including chrysolaminarin (Chrl), eicosapentaenoic acid (EPA), and fucoxanthin (Fx), which can be applied in aquaculture, human health foods, pharmaceuticals, and even cosmetics 31 .In addition, Nitzschia are common food organisms in aquaculture 32 .Therefore, if the shift from Microcystis dominance to Nitzschia dominance was achieved, the diatoms would provide more high quality food organisms for aquaculture, which is also beneficial to the material cycling and energy flowing in food web dynamics in algal blooming waters.

Experimental design
On the 8th of August, 2019, a thick Microcystis bloom was obtained from an aquaculture pond breeding mainly Megalobrama amblycephala, located in the Songjiang District of Shanghai, China.The bloom was dominated by Microcystis spp., particularly M. aeruginosa, and Microcystis made up 99.0% of the total biomass, while Anabaena accounted for 1.0%.On the 10th of August, 2019, the thickly bloomed Microcystis was transferred to transparent borosilicate 10 L glass jars (23 cm diameter, 35 cm high) in a greenhouse of 220 m 2 .A layer of plastic film was affixed on the glass of the greenhouse to block the sunlight.The experiment ended on the 9th September, 2019, which lasted 30 days.Tap water was used to dilute the thick bloom to obtain different biomass of Microcystis.The concentrations of total nitrogen (TN) and total phosphorus (TP) in the tap water were 1.088 and 0.011 mg L -1 , respectively.
Chlorophyll-a (Chl-a) concentration was chosen to reflect the biomass of the Microcystis bloom.There were three chlorophyll a (Chl-a) levels in six treatments (Table 1): low Chl-a level of 68.4 μg L -1 (L, L-E), medium Chl-a level of 468.7 μg L -1 (M, M-E), and high Chl-a level of 924.1 μg L -1 (H, H-E).Treatments L-E, M-E and H-E were enriched with the same inorganic nitrogen and phosphorus nutrients.Each treatment has three replicates.The initial TN, TP, and Chl-a concentration of the treatments is shown in Table 1.
During the experiment, treatments L-E, M-E and H-E were enriched with 2.1 mg/L nitrogen (N) and 0.3 mg/L phosphorus (P) every three days from 15th August on, which was in the form of NH 4 Cl and KH 2 PO 4 , respectively.Nine times of nutrients were enriched, with a total of 189 mg N and 27 mg P to satisfy the growth of algae.Thus, the concentration of TN and TP in treatment L-E was next to that in treatment M, and that in treatment M-E was close to treatment H. www.nature.com/scientificreports/Each jar was continuously aerated with a bubble stone.The aeration intensity was approximately 0.35 m 3 h -1 all the time, to maintain the suspension of Microcystis colonies as much as possible and prevent water splashing.No sediment was provided.Ultrapure water was used to replenish that lost by evaporation about every 3 days.

Shading rate in the greenhouse and its measurement.
The light intensity in the greenhouse was real-time changing, and the photosynthetically available radiation (PAR) were measured both inside the greenhouse and outdoors at about 11:00 h and 13:30 h, on a sunny day, the 14th of August, 2019, to show the shading rate in the greenhouse.PAR quantum (unit: μMol m -2 s -1 ) and PAR energy (unit: W m -2 ) was measured by a Spectrosense2 meter associated with a four-channel sensor (Skye Instruments, UK).It was measured three times in three minutes, and the average values were calculated to obtain the light transmittance, which was the ratio of the light intensity in the greenhouse to outdoors.Then shading rate was calculated as (%): shading rate (%) = 100 − transmittance (%).

Water quality and Chl-a measurements
During the experiment, water temperature (WT), dissolved oxygen (DO), salinity (Sal) and pH were measured at about 14:00 h every 2 days with a YSI multi-parameter water quality monitor meter (YSI professional plus, Yellow Spring Instruments, USA) in situ.The nutrient variables such as total nitrogen (TN), total phosphorus (TP), dissolved TN (DTN), dissolved TP (DTP), ammonia nitrogen (NH 4 + -N), and soluble reactive phosphorus (SRP) were measured every 6 days.Water for analysis of DTN, DTP, NH 4 + -N and SRP was filtered through 0.45 μm mixed fiber filters, which were washed with deionized water before using.Measurements of TN, TP, DTN and DTP were taken following the methods of Gross and Boyd 33 , while NH 4 + -N was determined by Nessler's reagent spectrophotometry and SRP was determined by molybdenum-antimony-ascorbic acid colorimetry 34 .A PHYTO-PAM (Waltz, Effeltrich, Germany) chlorophyll fluorescence meter with the software phytowin2.13was used to measure Chl-a.

Algae identification and counting
Algal samples were collected six times during the experiment, with a sampling frequency of every 6 days.The algal samples at the start and on day 6 were mixed samples of the aliquots from 3 jars, as well as those in treatments L and L-E on day 12, and the other samples were from the jars in 3 replicates.
To determine algal density, 50 mL water samples were preserved with 1% Lugol's solution and stored in darkness until analysis.For enumeration, two replicate aliquots were enclosed in 0.1 mL plankton counting chambers that were modified from the Palmer and Maloney design 35 .Most cells were observed at 400 × magnification by an optical microscope Olympus CX31 (Olympus, Japan), while large algal cells or colonies were observed at 100 × magnification.And some microphotographs were taken under 400 × magnification.The cells were mainly identified to the genus level as referenced by morphologies 36 .For the enumeration of cells in Microcystis colonies, subsamples were heated to 60 °C for 2-4 h to disintegrate the colonies.
Algal volumes were calculated based on cell density and cell size measurements.Calculation of cell volumes was according to their shape, and measurements of length, height, and diameter were obtained to calculate the volume.At least 30 algal units were measured to obtain the average cell volume for each genus or species.The conversion to wet weight biomass assumed that 1 mm 3 of volume was equivalent to 1 mg of wet weight biomass 29 .

Data analysis
Data comparison among the treatments was conducted with SPSS 24.0 software for Windows (Statistical Product and Service Solutions, IBM, New York, USA) using two-factor analysis of variance (ANOVA) (biomass × time) in the general linear model.To improve the homogeneity of variances, most data of each treatment were square root transformed before the comparison, and the cell density ratio of Microcystis to Nitzschia was log10 transformed, while the biomass ratio of Microcystis to Nitzschia was fourth root transformed 37 .The data were mainly shown as mean ± SD, except some algal wet weight was shown as mean.The comparison of the algal wet weight was for the data from days 12 to 30.The Least-significant difference (LSD) test was chosen for pairwise comparisons.

Light in the greenhouse
The glass and the affixed plastic film on the glass of the greenhouse shaded the sunshine, and the shading rate of the greenhouse was about 72 ~ 74% (Table 2).

Water quality
The WT in the greenhouse was real-time changing, and the range of WT was 20.5 ~ 34.6 °C during the experiment (Fig. 1a,b), and the WT at 14:00 was 2 ~ 5 °C higher than that at 08:30.DO ranged in 5.29 ~ 7.98 mg L -1 (Fig. 1c,d), showing that the Microcystis bloom was in aerobic instead of anaerobic condition.The pH values were in the range of 5.73 ~ 10.31 (Fig. 1e,f).The rank order from high to low in pH values was (L, M) > L-E > (M-E, H) > H-E (P < 0.05) in the morning, while that was M > L > L-E > (M-E, H) > H-E (P < 0.05) in the afternoon, showing the pH values decreased with the Microcystis biomass increase and the inorganic nutrient enrichment.www.nature.com/scientificreports/Sal ranged from 0.11 to 0.26‰ (Fig. 1g,h), and the rank order of Sal from high to low was H-E > M-E > L-E > H > (L, M) (P < 0.05) both in the morning and the afternoon, showing the Microcystis biomass increase and inorganic nutrient enrichment improved the Sal significantly (P < 0.05).
The comparison result showed the rank order from high to low in TN was H-E > (M-E, H) > (L-E, M) > L (P < 0.05), showing the N enrichment in treatment L-E and M-E increased the TN to the level in treatment M and H, respectively.The rank order for TP was H-E > M-E > (H, L-E) > M > L (P < 0.05), showing the P enrichment in treatment L-E improved the TP to the level in treatment H.

Chl-a
The Chl-a concentration in all the treatments gradually decreased until on about day 15, and remained relatively stable from days 18 to 30 (Fig. 3).The analysis result on Chl-a from the start to day 15 showed that Chl-a in treatment L was significantly lower than the other 5 treatments (P < 0.05).The Chl-a in treatments H and H-E was significantly higher than both treatments M and M-E (P < 0.001), and that in treatments M and M-E was significantly higher than both treatments L and L-E (P < 0.001).These showed the Microcystis biomass was the main factor affecting the Chl-a content.Then from days 18 to 30, there was no significant difference between treatments L, L-E and H in Chl-a content (P > 0.05), while they were all significantly lower than that in treatments M, M-E and H-E (P < 0.001).

Algal succession
At the beginning of the experiment, there were very few cells of Planktothrix, Chlamydomonas and Pseudanabaena besides the dominance of Microcystis.During the experiment, the phytoplankton was dominantly composed of Cyanobacteria, Chlorophyta, and Bacillariophyta, of which Cyanophyta and Bacillariophyta were the most dominant ones.The Cyanophyta were primarily composed of Microcystis, while the Bacillariophyta were mainly Nitzschia.There were a few green algae during the experiment, particularly in the low biomass treatments L and L-E, which were not counted.In this experiment, the Microcystis colonies gradually decomposed, and the genus Nitzschia, primarily Nitzschia palea grew in the Microcystis colonies.The changes in the cell density and wet weight of the two genera Microcystis and Nitzschia showed that the biomass of Microcystis gradually decreased while that of Nitzschia gradually increased (Fig. 4).And Nitzschia dominated in all the treatments from day 18 on (Fig. 4).
The analysis results showed the wet weight of Microcystis in treatments L and L-E was the lowest (P < 0.05), and that in treatments H and H-E was the highest (P < 0.05), showing the change in wet weight of Microcystis under aeration mixing depended on the initial Microcystis biomass.And the nutrients' enrichment in treatments H-E, M-E and L-E did not decrease the biomass of Microcystis, in comparison to treatments H, M and L, respectively (P > 0.05).www.nature.com/scientificreports/Basically, the higher the initial biomass of Microcystis in treatments L, M and H, the higher the biomass of Nitzschia in the later stage of the experiment (P < 0.05).Moreover, the biomass of Nitzschia in treatment H-E was significantly higher than treatment H (P < 0.05), while both of them were significantly higher than treatments M and M-E (P < 0.001).These showed the N and P enrichment was beneficial for the Nitzschia increase in the high biomass treatment H (P < 0.05).However, the nutrient enrichment in treatment M-E did not increase the Nitzschia biomass in comparison to treatment M, nor did it in treatments L-E to L (P > 0.05).
Both the cells' density ratio and biomass ratio of Microcystis to Nitzschia gradually decreased to be close to each other in all the treatments (Fig. 4).The density ratio of Microcystis to Nitzschia in treatment H-E can be near 1.0, and the biomass ratio of Microcystis to Nitzschia in treatment H-E can be lower than 1.0 (Fig. 4).The comparison result in density ratio and biomass ratio of Microcystis to Nitzschia showed the values in treatments H and H-E were the lowest (P < 0.05), and they were all significantly lower than the other four treatments (P < 0.05).The values in treatments L, L-E, M, and M-E were not significantly different from each other (P > 0.05).
Moreover, the Nitzschia cells were not in free-living forms but attached to the Microcystis colonies in almost all the treatments.The aggregates of Nitzschia cells in the Microcystis colonies in treatments H and H-E are shown in Figs. 5 and 6.

Discussion
In this experiment, the colonial Microcystis shift to diatom dominance in all treatments with aeration mixing in the greenhouse (Figs. 4, 5, 6); moreover, the initial colonial Microcystis biomass affected the Nitzschia biomass (Fig. 4).This was similar to our earlier results in Wang et al. 27 , which showed that the biomass of bloom-forming colonial Microcystis affected its response to aeration disturbance, and diatom Nitzschia appeared in the Microcystis colonies when the initial Chl-a of the bloomed Microcystis was 346.8 μg L -1 .The Nitzschia biomass in treatments M, M-E, H, H-E was much higher than that in Wang et al. 27 , showing the initial colonial Microcystis biomass in treatments M, M-E, H, H-E was more suitable to promote the diatom succession.Thus, in comparison with the results in Wang et al. 27 , more diatom Nitzschia shift from the Microcystis colonies when the Microcystis colonies' biomass was 468.7 and 924.1 μg L -1 in Chl-a.And it can be inferred that the Microcystis biomass level between 468.7 and 924.1 μg L -1 in Chl-a was also suitable to achieve the shift from Microcystis dominance to Nitzschia dominance.
The fact that the initial Microcystis biomass affects the diatom succession and Nitzschia cells adhere to Microcystis colonies is very interesting.Similarly, the abundance of Microcystis affected the algal shift under hydrodynamic disturbance at Hartbeespoort Dam in South Africa 21 .When Microcystis abundances were high, the diatoms Cyclotella meneghiniana and Melosira (syn.Aulacoseira) granulata and the cryptophytes Chroomonas sp. and Cryptomonas sp.occurred more frequently 21 .On the contrary, when Microcystis was rare, some green algae tended to increase across a broad spectrum of temperature and nutrient conditions 21 .
Moreover, many studies have found that hydrodynamic mixing caused the algal shift from cyanobacteria, particularly Microcystis bloom, to diatom or/and green algae dominance [16][17][18]22,38 . Mixin shifts the Microcystis blooms from Lake Taihu, China, to the dominance of diatoms and green algae 20 .Diatoms such as Asterionella, Fragilaria, and Staurastrum were favored by deep mixing, which hampered the cyanobacteria (Microcystis and Anabaena) 39 .The growth of Microcystis in Nieuwe Meer Lake of the Netherlands was reduced by artificial disturbance 17 , and high-intensity hydrodynamic disturbance changed the Microcystis bloom to a diatom and green algae bloom 16,18 .The cyanobacteria bloom in Ford Lake shifts to a diatom bloom with artificial disturbance 19 .Continuous hydrodynamic mixing weakened the dominance of Microcystis, which was beneficial for the other algae 26 .Artificial aeration replaced the dominant bloom-forming cyanobacteria with diatoms in a small tropical reservoir 40 .
However, the roles of cyanobacteria biomass on the shift from cyanobacteria dominance to non-cyanobacteria dominance under artificial mixing were not considered by Visser et al. 22 , and how the algae reacted to turbulence mixing is not fully understood 23,41 , although that turbulence mixing plays a vital role in influencing the algal growth rate is widely accepted.The reasons for hydrodynamic turbulence favoring the dominance of diatoms and green algae other than buoyant cyanobacteria were summarized by some researchers 20,22 , which include buoyancy regulation, competition for nutrients, competition for light, sedimentation losses, and so on.In this experiment, aeration mixing directly caused the aggregates of Microcystis colonies to suspend and keep rolling in the water.And the indirect impact includes the influence from changes in nutrient level, suspended particulate matter, light shading, DO, and so on.
Many factors affected the algal shift from Microcystis dominance to diatom dominance under aeration mixing, and nutrients are one of the important factors.The decrease of Chl-a and increase of NH 4 + -N and SRP in all the treatments expressed the decay of the Microcystis colonies.Moreover, the higher the Microcystis biomass, the more dissolved nutrients release (Fig. 2).These dissolved nutrients were the material basis for the algal shift.Similarly, algal decomposition can release some nutrients to support other algae 24 .In treatments L, M, and H with no inorganic nutrients enrichment, the released nutrients from Microcystis supported the algal shift to Nitzschia dominance (Figs. 2 and 3).The inorganic nutrient enrichment in treatment H-E improved the biomass of Nitzschia in comparison with treatment H; on the other hand, the enrichment in treatment M-E did not improve the biomass of Nitzschia in comparison with treatment M (Fig. 4).These indicated that it's not always the case that the higher the concentration of inorganic nitrogen and phosphorus nutrients, the better for the diatom succession.
Light is also an important factor affecting the algal shift.In this experiment, the aggregates of Microcystis colonies can be physically mixed into the water column with the aeration mixing, which produces "self-shading".And it is sure that the higher the Microcystis biomass, the lower the light intensity in the jars.The biomass of colonial Microcystis affected the light intensity in the jars as well as the nutrients' level.Moreover, the greenhouse www.nature.com/scientificreports/ was in low light with a shading rate of about 72~74% in comparison with the light outdoors.It's probable that the low light satisfied the light demand for the diatom growth, as diatoms and green algae are considered to be better adapted to fluctuating light conditions in comparison to buoyant cyanobacteria 18 .Diatoms are also considered to prefer to low light conditions, and high irradiance contributed to the decline of the spring diatom 42 .Huisman et al. 18 established a light competition theory, which meant that light instead of nutrients would be the limiting www.nature.com/scientificreports/factor for algae growth in highly eutrophic conditions with full mixing.Mixing affected the light competition between buoyant and sinking phytoplankton species in eutrophic waters 18,43 .Moreover, the algal response to aeration mixing depended on the algal group's characteristics.Compared to other algal groups, cyanobacteria were regarded as relatively more sensitive to hydrodynamic turbulence 44 .Some information for the development of phytoplankton communities in turbulent water was provided by Kang et al. 41 , which studied the reconstruction of phytoplankton under the dual effects of turbulence and suspended particulate matter, and found that the turbulent treatments promoted the shift from Microcystis sp.dominance to Scenedesmus sp. and Chlorella sp.dominance.
The shift from Microcystis dominance to diatom dominance in this experiment is very distinctive, in which the N. palea cells formed aggregates in the mucilaginous sheath of Microcystis colonies instead of free-living (Figs. 5 and 6).The aggregation of cells in Microcystis colonies is due to the mucilaginous sheath, which mainly consists of EPS (extracellular polymeric substances) 45,46 .In comparison to the decomposition of a high biomass of Microcystis colonies in anoxic or anaerobic conditions 47 , the decomposition under aerobic conditions was much slower 27 .The aerobic conditions caused by the aeration mixing slowed down the decomposition of Microcystis colonies in comparison with anoxic or anaerobic conditions, in which the EPS of Microcystis colonies provided a physical medium for the diatoms' aggregation under aeration mixing (Figs. 5 and 6).
Similarly, diatoms coexisted with Microcystis in African freshwater lakes, as well as the filamentous cyanobacterium Pseudoanabaena sp.coexisted with Microcystis colonies by adhesion way 48 .Diatom Nitzschia is a common genus that forms aggregates in marine snow 49 , and it's considered that the diatoms are accompanied by the production of a large number of EPS.Transparent exopolymer particles (TEP) are a large class of EPS with high stickiness that promotes the formation of diatoms aggregates in marine snow 49,50 .However, the EPS form the diatom aggregation in this experiment comes from the Microcystis colonies, which was not released by the diatoms.This experiment not only provides an appropriate biomass of Microcystis bloom for the shift from Microcystis dominance to diatom dominance, but also shows that the EPS of the Microcystis colonies provides aggregating matrix for the diatoms, which is similar to the formation of marine snow with diatom aggregation.
Moreover, the phenomenon of diatoms adhering to algal blooms also occurs in the marine red tide species, Phaeocystis.In marine red tide research, diatoms utilize Phaeocystis colonies not only as habitat, but that they were able to utilize the colonial matrix as a growth substrate 51 .Numerous studies have found that diatoms, including Nitzschia can attach to and grow on Phaeocystis colonies 52,53 , and the mechanism may be similar to the Nitzschia aggregates in this experiment.

Conclusions
This study showed that the algal shift in colonial Microcystis blooms under aeration mixing was affected by the initial biomass, and when the initial Microcystis biomass was appropriate, such as at Chl-a levels of 468.7 μg L -1 and 924.1 μg L -1 , and the levels between them, the Microcystis dominance could shift to diatom dominance, particularly Nitzschia palea dominance.During the algal shift under the aeration mixing, the colonial Microcystis decayed and released soluble inorganic nitrogen and phosphorus; however, enrichment of more soluble inorganic nitrogen and phosphorus was beneficial for the Nitzschia increase in the high biomass treatment alone.The Nitzschia cells were in aggregates with the Microcystis colonies instead of free-living, and the mucilaginous sheath of the Microcystis colonies provided the physical medium for the aggregation.This study found for the first time that Microcystis blooms could shift to Nitzschia dominance in aggregates.This experiment provided a method to control and manipulate Microcystis blooms to diatom dominance through continuous aeration mixing to proper biomass of Microcystis colonies.The shift to diatoms dominance would provide more high quality food organisms for aquaculture and be beneficial to the material cycling and energy flowing in food web dynamics.

Figure 3 .
Figure 3. Changes in Chl-a of each treatment during the experiment.

Figure 5 .
Figure 5.Some representative pictures on the aggregates of Nitzschia cells in Microcystis colonies in treatment H (a-d) and treatment H-E (e-h) on day 20.

Table 1 .
The concentration of Chl-a, TN and TP in each treatment at the start of the experiment, showing whether the treatment was nutrient enriched (mean ± SD).

Table 2 .
The PAR both in the greenhouse and the outdoor, and the calculated shading rate of the greenhouse on August 14, 2019.